Intraocular pressure is generally measured using a tonometer, which is placed on the surface of the cornea and measures the elasticity of the latter using various methods (Goldmann's tonometer, Schiotz's tonometer, etc.). Two of the most commonly used principles are measurement of the force required to applanate a certain area of the surface or measurement of the diameter of the area applanated by a known force. These methods require the patient's cooperation and cannot be applied without general anaesthesia to small children, persons suffering from dementia, or animals.
Methods such as U.S. Pat. Nos. 5,148,807, 5,279,300 and 5,299,573, have also been developed, in which the surface of the cornea is not touched, the intraocular pressure being measured instead by means of a water or air jet or various waves. Such methods are technically complex and thus expensive. Meters operating on the air jet principle are widely used by opticians, but their expense has prevented their extensive use by general practitioners.
U.S. Pat. No. 5,176,139 also describes a method in which a freely-falling ball is dropped onto the eyelid and the height of the ball's rebound is measured.